In his 1752 Treatise on the Theory and Practice of Midwifery, celebrated Scottish obstetrician William Smellie attributed a newborn’s arm paralysis to “prolonged compression,” itself the result of Smellie’s own obstetric maneuvers. This would be the first medical description of an injury to the brachial plexus, a network of nerves in the shoulder and arm. Smellie’s causal explanation remained dominant for centuries. When medical researchers attempted to explain an infant’s brachial plexus injury, they always returned to Smellie’s initial diagnosis: injuries to the brachial plexus are invariably caused by doctors, who had intervened in difficult deliveries without sufficient caution.
More recent analysis has called this conventional wisdom into doubt. Today, researchers believe that brachial plexus injuries are caused by the interplay of wide-ranging factors, from maternal health conditions to large babies. Inexpert doctors are only one part of the puzzle.
Brachial Plexus Injuries: The Search For Causes
Where long-term injuries are concerned, however, even this nuanced picture has proved unsatisfying. Cutting-edge research has shown that “established” risk factors don’t apply to most children who sustain permanent brachial plexus impairments.
Instead, the overwhelming majority of long-term disabilities can be traced back to a single delivery complication, shoulder dystocia, a medical emergency that often requires obstetric intervention. In contrast to the conventional wisdom, this research puts the spotlight back on individual doctors and dangerous intervention techniques.
How Many Babies Suffer Brachial Plexus Injuries?
Neonatal brachial plexus injuries are unfortunately common, even given today’s most advanced medical systems. The majority of studies, including this 2011 report from researchers at the Texas Nerve and Paralysis Institute, suggest that infants will suffer neonatal brachial plexus injuries in between 0.38 to 5.8 out of 1,000 live births.
While those numbers may imply that brachial plexus injuries are relatively rare, damage to the brachial plexus nerves undoubtedly accounts for a high proportion of birth injuries overall. In fact, neonatal brachial plexus injuries may represent up to 5% of all birth injuries sustained in the United States, according to Kathleen Benjamin, a Registered Clinical Nurse at Children’s Hospital Colorado. Thankfully, many of these injuries will heal spontaneously, allowing a child to regain full function in the affected arm. Permanent impairments, however, are not uncommon. Some children will be left with life-long disabilities.
Explaining Brachial Plexus Injuries
To prevent these injuries from occurring in the future, we need a solid understanding of why they happen. For more than a century, the medical community was fairly certain that it had found a comprehensive explanation: brachial plexus injuries occur when obstetricians pull on a child’s head, arm or shoulder, straining the nerves.
Newer research, however, has complicated this simplistic picture. In the medical community’s developing narrative, perhaps best represented by the Journal of the American Osteopathic Association, direct obstetric intervention is only one factor that can contribute to injuries of the brachial plexus.
Do “Risk Factors” Tell The Whole Story?
Alongside manual or instrumental traction, many doctors now place pre-existing maternal health complications and the inherent delicacy of nerves as contributing causes.
Neonatal brachial plexus injuries are far more likely to occur during “difficult” deliveries, as described by mothers and attending medical professionals. In a 1997 Swedish study, for example, mothers of children with brachial plexus injuries described the birth process as “difficult” in 77% of cases. Only around 20% to 27% of women in control groups described their deliveries in similar terms.
Established Risk Factors For Brachial Plexus Injury
“Difficult,” of course, can have a wide range of connotations, both objectively and subjectively. To explain the causes of brachial plexus injuries, researchers often point to a short list of “established” risk factors:
- Fetal macrosomia (above-average infant birth weight)
- Instrument-assisted delivery (use of forceps or vacuum extraction)
- Downward traction of the fetal head (through obstetric intervention)
- Breech delivery (an abnormal fetal presentation in which the feet or buttocks enter the birth canal first)
- Cesarean section
- Maternal diabetes
- Multiple pregnancy
- Oxygen deprivation
As you can see, some of these traditional risk factors are enough to make a delivery difficult. Abnormal fetal presentations, for example, can significantly complicate childbirth, extending labor periods, increasing the risk of birth asphyxia and requiring potentially-dangerous levels of effort from a mother. Other risk factors, like instrumental delivery, are better described as an obstetrician’s response to a delivery that has become difficult.
In any event, none of these risk factors are foolproof indicators that a child will sustain a brachial plexus injury. In fact, at least one study, in which doctors at the University of Cincinnati reviewed over 11 million US births, found that a majority of children who sustained brachial plexus injuries had no known risk factors.
Calling Risk Factors Into Question
This isn’t to say that risk factors mean nothing. Doubtless, medical professionals must remain vigilant, both before and during childbirth, carefully watching for signs of a complication and then quickly implementing appropriate treatments.
Indeed, some pregnancy complications can be good predictors that an infant is at risk of sustaining a brachial plexus injury. In the Cincinnati study, for example, “exceptionally large” babies, those weighing more than 9.9 pounds at birth, were 14 times more likely to sustain injuries to the brachial plexus. Shoulder dystocia, however, was far-and-away the most likely cause of an injury to the brachial plexus, increasing the risk by up to 100 times.
Long-Term Brachial Plexus Injuries Defy Traditional Risks
As we’ll learn in a moment, shoulder dystocia is not itself a leading cause of brachial plexus injuries. It is, however, a pregnancy complication that almost always requires immediate medical intervention. In cases of shoulder dystocia, an infant’s shoulder becomes lodged behind the mother’s pelvis, preventing further progress through the birth canal. Without appropriate treatment, the child can be deprived of oxygen. Oxygen deprivation, or birth asphyxia, is a leading cause of brain damage in newborn infants. Thus shoulder dystocia is a true medical emergency, one that usually requires prompt and careful intervention.
How to intervene successfully, while preventing undue strain to a child’s brachial plexus nerves, is another matter. Numerous intervention techniques have been considered, but regardless of an obstetrician’s choice, cases of shoulder dystocia are far more likely to result in permanent brachial plexus injuries. By the same token, brachial plexus injuries that are unrelated to shoulder dystocia have a much better prognosis.
As physicians at the Texas Nerve and Paralysis Institute report, up to 18% of children born with brachial plexus injuries nationwide experience shoulder dystocia during delivery. That statistic, however, doesn’t take the the severity of the injury into account. When a child’s prognosis is considered, cases of shoulder dystocia appear to increase the likelihood of permanent disability to an astonishing degree. In their own study, which reviewed 241 cases of permanent brachial plexus injury, the Texas researchers found a stunning rate of shoulder dystocia. No less than 97% of the children with permanent brachial plexus impairments had experienced shoulder dystocia.
Other researchers have confirmed these results, finding that cases involving shoulder dystocia are far more likely to result in permanent paralysis. In a review of 63 infants with permanent Erb’s palsy, an injury in which the upper brachial plexus nerves are damaged, shoulder dystocia contributed to 94% of the injuries.
Shoulder Dystocia: Key To Permanent Impairments?
Most importantly, these same studies have found that, beyond shoulder dystocia, the “established” risk factors for brachial plexus injuries are not implicated in cases of permanent impairment. In the Texas study, for example, no link could be found between a baby’s birth weight and their likelihood of sustaining permanent injuries. Likewise, maternal diabetes and high maternal weight, two other commonly-cited risk factors, also appeared to be unrelated to the prognosis of a child’s injuries.
In short, a growing body of evidence suggests that, while “established” risk factors may prove important in predicting whether or not a child will sustain brachial plexus injuries during delivery, they cannot tell us how likely a child is to develop permanent impairments. Shoulder dystocia appears to be the only traditional risk factor tied to life-long disabilities, occurring in nearly all deliveries that lead to permanent brachial plexus impairments.